The present invention relates to a method for regulating braking torque generated by a braking system in a wheeled vehicle according to the preamble of claim 1, and to an arrangement for regulating braking torque generated by a braking system in a wheeled vehicle according to the preamble of claim 11. The invention further relates to the use of pressure regulating means in a braking system of a wheeled vehicle to restrict brake torque generated by said braking system.
The invention is primarily intended for, but not restricted solely to, applications in braking systems of commercial vehicles such as trucks and lorries.
Electronic control is being employed to an ever increasing extent in vehicles to ensure optimal efficiency and reliability of various operational systems. A recent development in the truck industry is the availability of electronic braking systems (EBS). Although compressed air is still used to apply the brakes in EBS, electronics are used to command pressure-regulating modules to pressure the brake cylinders with compressed air to thereby engage the brakes. At least theoretically, by being able to monitor various forces, load changes and individual braking rates, EBS can continuously modify the braking on each wheel to thereby provide optimal distribution of the braking force. Anti-lock braking (ABS) and traction-control (ASR) functions can also be integrated in the EBS.
As is described in WO 91/09758, EBS can be employed to adapt the brake pressure supplied to the brakes of a trailer to that supplied to the brakes of the hauling truck.
Irrespective of whether a truck is fitted with EBS, during an emergency stop the brake torque generated by the brakes rapidly rises to a short duration peak value before leveling off to a lower value. This effect is illustrated in FIG. 1 for a drum brake system in which brake torque is plotted as a function of time for emergency braking from three different speeds. Thus, curve A represents emergency braking from 20 km/h, curve B from 60 km/h and curve C from 100 km/h. It is apparent that the maximum value of peak brake torque arises when emergency braking from relatively low speed.
The use of disc brakes on commercial vehicles is becoming more widespread. Generally, disc brakes are lighter than corresponding drum brakes, thereby contributing to reduced unsprung weight. Furthermore, they have the ability to dissipate heat more effectively than drum brakes, which implies that they are often less prone to brake fade from high speeds. In order to slow down a heavily laden truck from high speeds, the disc brake actuators must be capable of exerting high pressure on the brake pads. As is apparent from FIG. 2, this high braking pressure results in even greater peak values for the brake torque than those attained with a drum brake system when emergency braking from corresponding speeds.
The high peak values of brake torque when emergency braking from low speed, and as represented by curve D in FIG. 2, create significant problems for the truck manufacturer. Firstly, the vehicle suspension components and axles, particularly at the front of the vehicle, must be dimensioned to be able to withstand these peak braking torques. This implies a substantial increase in weight and manufacturing costs. Secondly, the peak braking torque may be sufficient to activate the ABS system even if the truck is travelling on a high-friction surface such as dry asphalt. On such surfaces and at speeds below say 25 km/h, a shorter braking distance is in fact attained if the ABS does not cut in. Thirdly, the severe jolt which is transmitted through the truck upon emergency application of the brakes at low speeds implies that the cargo has to be more securely tied down than would be the case if such peak brake torques did not arise. This means of course that extra time and expense is involved in equipping the truck with adequate cargo-retaining means.
It is therefore an object of the present invention to provide a method and an arrangement for overcoming the problems associated with peak brake torque arising during emergency braking from low speeds.
This object is achieved by the method according to claim 1 and by the arrangement according to claim 11.
Preferred embodiments of the method and arrangement are detailed in the respective dependent claims.
The invention further provides for the use of pressure regulating means in a braking system of a wheeled vehicle to restrict brake torque generated by said braking system to a predetermined value below a maximum value when said vehicle is travelling below a predetermined speed when said braking system is initially actuated.
In accordance with the invention, when emergency braking from a speed below a predetermined value, braking pressure is restricted to a predetermined value which lies below the maximum value of pressure which is otherwise attainable above the predetermined value of speed. In this manner, the peak brake torque value when emergency braking from low speeds will be relatively lower without compromising desirable high braking pressure when emergency braking from higher speeds. This implies that even more effective brakes can be fitted to a vehicle without having to strengthen existing suspension components and mountings.